1. Field of the Invention
This invention relates to a motor-driven power steering system for a vehicle adapted to assist the operator-induced steering operation by means of the rotating force of a motor.
2. Description of the Prior Art
In the past, a conventional power steering system of this type, as schematically illustrated in FIG. 10, has hitherto been known in which a steering wheel 1 is operatively connected through a steering shaft 2 and a first rack and pinion gear T.sub.1 including a first pinion 5 and first rack tooth portion 6a with a steering rack 6 which is connected at its opposite ends with a pair of steerable road wheels (not shown) through a pair of tie rods 8a and 8b so that when the steering wheel 1 is turned by an operator, the steerable road wheels (not shown) are appropriately steered in accordance with the steering motion of the steering wheel 1 imparted by the operator. On the other hand, the steering rack 6 is operatively connected with a motor 13 through a second rack and pinion gear T.sub.2 including a second rack tooth 6b and a second pinion 18 and a speed-reduction gear R so that the driving force of the motor 13 is transmitted through the speed-reduction gear R and the second rack and pinion gear T.sub.2 to the rack 6 so as to assist the steering operation of the steering wheel 1 imparted by the operator. The motor 13 is electrically connected with a battery 11 through a control unit 9 and a key or ignition switch 12 so that it is energized by the battery 11 under the control of the control unit 9. The control unit 9 is input with control signals from a steering-torque sensor 3 and a vehicle-speed sensor 10 so as to appropriately control the operation of the motor 13 and the switching clutch 17 on the basis of the steering torque and the vehicle speed measured.
The power steering system as constructed above has the following problems. Specifically, when the vehicle is travelling on a bad road having many irregularities at a relatively low speed, the steering wheel 1 is sometimes forced to turn by the road surface irregularities through the power steering system against the operator's will, thus impairing steerability. Further, when the vehicle is accelerated or decelerated, the steering force required to turn the steering wheel 1 to a predetermined angle varies, making it difficult to maintain a desired course because of the resultant inaccurate steering response. Moreover, if the power steering operation is effected when an alternator, acting to supply electricity to the battery 11, is not generating power, or when the battery 11 is not sufficiently charged, the battery 11 will be run down or exhausted in a short time.